In the prior art, there are several different types of frames for dynamoelectric machines, such as AC and DC motors and generators, several types of end members known as end bells, and several types of brush holders. Frames have been cast in a hollow tubular shape and machined to provide a smooth inner surface of predetermined diameter. Frames have been cast to have a predetermined contour on the inner surface, and partially machined so that a portion of the length of the frame is adapted to receive field coils or magnets, or produced by pouring a plastic material around field magnets and flux path members. Brush holding channels are attached to or made integral with an end bell, which also serves to support the shaft of the dynamoelectric machine adjacent the contact area, such as a commutator surface or slip ring surface of the rotating armature.
As is well-known, it is difficult to renew brushes mounted in an end bell of a dynamoelectric machine, since the machine must be disassembled to remove the end bell, generally by removing through-bolts from the machine, and the end bell reinstalled with new brushes. Retaining the brushes in a retracted position until the end bell has been reinstalled, with the brushes adjacent the commutator or slip ring, and then releasing them, is a difficult task at best, even when using the thin resilient special tools used for initial assembly of the machine. Of course, the replacement brushes can be severely damaged and broken in this attempt. For this reason, dynamoelectric machines with brushes in an end bell which must be removed to replace brushes is generally considered very difficult to repair. Large motors with deep end bells are usually provided with access plates which can be removed to gain access to brushes, so that brushes can be replaced without removing the end bell. Smaller motors, and those with flat or shallow end bells cannot be provided with access openings.
To make a small dynamoelectric machine easily repairable, cartridge-type brush holders are used. Typically, these brush holders have a generally cylindrical plastic body surrounding a rectangular tube which accepts a rectangular brush, and accepts a threaded cap which makes electrical contact with the rectangular tube. The brush has a lead which terminates in a small flat plate and a spring encircling the lead between the brush and the flat plate. In use, the brush is inserted in the rectangular tube of the brush holder, and the cap presses the metal plate into electrical contact with the rectangular tube. Internal electrical connections to the brushes are made to the rectangular conductive tube. Thus, renewing the brushes is a relatively simple task. However, the motor frame must have a thick enough section to retain the cartridge-type brush holder. For example, a cast motor frame may be machined adjacent the armature, and not machined adjacent the commutator to leave the frame thick in that area, and a cartridge brush holder having a retaining flange and a threaded area for a retaining nut on the other is placed in a bore through the frame.
Alternately, to conserve material, the frame may be cast with thick areas only where the cartridge brush holders will be used. A bore for the cartridge brush holder is made, and threaded bores adapted to receive set screws are made in the direction of the axis of the machine, intersecting the bores. The brush holders are inserted, and retained by set screws tightened against the body of the brush holder, and locked in place by varnish or the like.
Typically, modern motors of greater than two inches (5.08 cm) and approximately eight inches (20.4 cm) are produced with a tubular iron or steel frame, serving both as a mechanical enclosure and a path for the magnetic flux of wound field poles or field magnets, as thin as is practical for the flux density necessary. This frame is provided with relatively flat, cost-efficient end bells or plates, one of which carries brushes and a shaft bearing, the other carrying only a shaft bearing, the machine being assembled with through-bolts. As will be apparent, it is difficult to service such a machine to replace its brushes, one approach being complete disassembly of the machine, placing the armature contact surface between the brushes on the end bell, and inserting the armature and end bell into the frame as an assembly. This cannot reasonably be done with wound-field machines, since it would be necessary to reconnect the connections between brushes and field coils at reassembly, a task which cannot be accomplished with the brush-holding end frame on the machine frame, because the wound poles cannot be removed after original assembly. Alternately, the end plate and brushes may be placed over the armature contact surface. This is often a long and tedious task, due to the difficulty of holding spring-loaded brushes clear of the armature contact surface when the brushes themselves are nearly inaccessible.
The instant invention overcomes these and other deficiencies of the prior art.